1. Field of the Invention
This invention relates generally to a method of communicating between devices in a telecommunication system and, more particularly, to a communication protocol for asynchronous data communications.
2. Description of the related art including information disclosed under 37 CFR 1.97-1.99
Telecommunication systems having a multiport switch controlled by a central processing unit to interconnect telephone units of an external telephonic network with an internal network of agent telephonic units connected with the switch are well known. Examples of such telecommunications systems are shown in patent application U.S. Pat. No. 5,268,903 of Jones et al. entitled "Multichannel Telephonic Switching Network With Different Signaling Formats and Connect/PBX Treatment Selectable For Each Channel", filed Oct. 2, 1991; U.S. Pat. No. 5,140,611 of Jones et al. entitled "Pulse Modulated Self-Clocking and Self-Synchronizing Data Transmission and Method for a Telephonic Communication Switching System", issued Aug. 18, 1992; U.S. Pat. No. 5,127,004 of Lenihan et al. entitled "Tone and Announcement Message Code Generator for a Telephonic Switching System and Method", issued Jun. 30, 1992 and U.S. Pat. No. 4,627,047 of Pitroda et al. entitled "Integrated Voice and Data Telecommunications Switching System", issued Dec. 2, 1986.
In such telecommunication systems, cathode ray tube (CRT) terminals are often connected to the central processing unit to receive data from the telecommunication system to display at a remote location. The CRT terminals used in these known systems are "dumb" and therefore have no processing or programming capabilities. All processing is done at the central processing unit of the telecommunication system. Disadvantageously, this limits the processing capabilities of the telecommunication system.
Therefore, in many known telecommunication systems distributed computer processing schemes are used in which the processing of information is performed at remote computing devices and the resultant data is sent to the central processing unit. Distributed processing is often desirable, since each remote computing device can be devoted to perform specific applications, and efficient use of computer processing power is achieved. However, it is nearly impossible for computer based devices from different vendors to communicate without a standard set of rules to follow for transmitting data.
Accordingly, in such telecommunications systems it is also known to communicate according to a preselected protocol which specify how the communicating devices initiate calls, terminate calls and transmit data between each other. Only computer based devices which have the same communication protocol or method of communicating are able to communicate with each other. Thus, the International Standards Organization (ISO) has established the Open Systems Interconnection (OSI) standard protocol in order to develop an internationally accepted standard framework for communication between data communication devices made from different vendors.
Referring to FIG. 1, the OSI reference model 20 of the prior art is arranged into seven layers. The seven layer OSI model refers to a collection of network processing functions that together comprise a set of rules and standards for successful data communication. Layer one is the physical layer 22, which performs the physical control of sending data over communications lines. Layer two is the data link layer 24, which provides the functional and procedural means to establish, maintain and release data lines between network entities. Layer three is the network layer 26 and is used for determining how data is transferred between computing devices. Layer four is the transport layer 28 which defines the rules for information exchange between devices. Layer five is the session layer 30 which manages the dialogue between the communicating devices. Layer six is the presentation layer 32 which masks the differences of the varying data formats between the dissimilar devices. Layer seven is the applications layer 34 which contains the functions for particular applications services.
X.25 is a known protocol commonly used in packet switching networks which conforms to the OSI seven layer reference model. The X.25 protocol is frequently used to enable many types of mainframe computers, minicomputers and large scale microcomputers to communicate with each other. High level Data Link Control (HDLC) is another known standard protocol. The HDLC protocol, developed by the International Standards Organization, consist of layer two of X.25. Protocols X.25 and HDLC are synchronous protocols designed for communication between devices from different vendors which transmit data synchronously.
Unfortunately, asynchronous devices, such as personal computers and workstations, typically do not support the X.25 and HDLC protocols. Therefore, these known protocols are not capable of providing data communications between a synchronous device and an asynchronous device. Personal computers having asynchronous interface characteristics are desirable as remote computing devices used in conjunction with a main central processing device in a telecommunication system because of their computing capabilities. A personal computer unlike a "dumb" CRT terminal, is capable of processing functions thereby relieving the computing load from a main central processing device in a system using distributed computer processing. Disadvantageously, the known protocols do not enable communication between a device which transmits data asynchronously such as a personal computer, and a device which transmits data synchronously.